1. Technical Field
The present invention relates to an optical power source for array illumination and, more particularly, to an externally modulated laser configuration.
2. Description of the Prior Art
Many advanced photonic systems such as optical computing and free-space photonic switching are based upon arrays of symmetric self-electrooptic effect devices (S-SEEDs). In these systems, the S-SEEDs function as detectors, logic elements, or modulators. Each system relies on arrays of light beams (modulated by the S-SEED arrays) to transmit information between components of the system. One method for producing the required light beam array is discussed in an article entitled "Binary phase grating generating even numbered spot arrays" by R. L. Morrison et al. appearing in the 1989 Technical Digest of the Optical Society of America, Annual Meeting (Vol. 18). The Morrison et al. article describes the beam array generator as comprising a laser diode source and a spot array generator mechanism, such as a grating and appropriate lensing. In operation, a collimating lens is coupled to the output of the laser diode, followed by a grating for producing a diffraction pattern related to the size and periodicity of the grating, and a Fourier transform lens disposed a focal length above the plane associated with the location of the spot array at the output of the grating. The optical Fourier transform lens functions to change the wavelength-dependent diffraction pattern into a spatial array (one- or two-dimensional) of beams, or spots.
Unfortunately, the above-described array generator mechanism requires stringent tolerances on the laser wavelength, since the spatial displacement of the light spots is a function of wavelength. For example, if the wavelength is too short or too long, the spot spacing is either reduced or expanded, respectively, such that extreme spots are no longer aligned with the underlying photosensitive device (e.g., S-SEED, 1- or 2-dimensional opto-electronic integrated circuits, photodiodes, etc.). As array size grows, wavelength control becomes more critical. Additionally, the photo-sensitive area of a SEED is concentrated within each cell to improve its performance. Therefore, the beam array must not only be wavelength-controlled, but physically aligned within the photo-sensitive area of the receiving device.
As photonic-based systems increase in size and speed, the need for higher power sources grows accordingly. In particular, the operating speed of a digital system is determined by the energy required to switch the state of an individual logic device and the capacity of the power supply. One such digital system may comprise a photonic switch capable of handling over a thousand channels modulated at about 100 MHz (on each channel). Such a system may comprise several modules with at least one S-SEED devoted to each channel. Given a switching energy of approximately one pJ and an optical efficiency of about 10% (for relaying the light from the laser to the S-SEED array and then on to the next module), a laser of about 1000 mW power would be required for each module.
A need remains in the art, therefore, for a light source capable of generating a spot array which is stable with wavelength and provides sufficient power for photonic system applications.